Chemistry of Stable Carbenes and «Green» Technologies

Abstract

The fundamental researches in the chemistry of stable carbenes carried out by the L.M. Litvinenko Institute of Physical, Organic & Coal Chemistry of the NAS of Ukraine (IPOCC) over the last decade and applications in the field of «green» technologies based on the results of these researches have been described. Carbene versions of Claisen ester condensation with the formation of zwitterionic compounds, the Leuckart-Wallach reaction with the autoreduction of carbenoid azolium salts, the Hofmann cleavage of aminocarbene insertion products, induced tandem autotransformation of 1,2,4-triazol-5-ylidenes into 5-amidino-1,2,4-triazoles have been discovered. New carbene reactions of addition, deesterification, oxidation, and complex formation have been found. Effective methods for obtaining stable carbenes and carbenoids have been suggested. New types of carbenes, benzimidazolylidenes, superstable conjugated bis-carbenes, and carbenoids have been synthesized. The existence of hypernucleophilic carbenes has been theoretically predicted and experimentally confirmed. Prospects for the use of carbenes and their derivatives, in particular, carbene complexes of transition metals in the catalysis of organic reactions and the search of biologically active compounds have been outlined. K e y w o r d s: heteroaromatic carbenes, synthesis, properties, transformations, catalysis, biological activity, and «green» chemistry.

Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

“…These undergo deprotonation with strong bases even in aprotic media, thereby resulting in ring closure to 1-cyanoamіdines. 209 1,2,4-Triazolіum salts with electron withdrawing groups (3-nіtro and 4-(2,4-dinіtrophenyl)) have been used for the preparation of rhodium complexes (structures 42C). 210 Compounds of this type could be some of the least electron donating carbenes known.…”

“…Included are significant increases of their basicities and nucleophilicities by charge transfer to the carbene center via the conjugation chain. 21,212 This outcome is particularly clear in the cases of oxido-, sulfido-, imido-, phosphido-and other related structures of the type 45А, all of which exert their influence via an aromatic ring (Scheme 45). The phenoxides and phenothіoxides themselves do not possess high basicities and proton affinities (PAs).…”

“…These have been termed "hyperbasic and hypernucleophilic" carbenes (HBNC). 21,212 The anionic centers can be either exocyclic or endocyclic. According to DFT calculations (carried out at the B3LYP5/6-31G/RHF level), the proton affinities PA for carbene systems with exocyclic anionic centers range in value from 330 to 460 Kcal/mol, and the affinities for copper (I) and potassium cations are 220-424 and 92-230 Kcal/mol, respectively (Scheme 55).…”

“…Although in situ transformations are sometimes able to do surprising things (see, for example, the recently described 31 formation of carbene complexes of copper(I) from azolium salts and metallic copper), individual carbenes not only catalyse organic reactions but also open up new possibilities for organic synthesis. 32,33 The present article is concerned with the syntheses, structural studies and transformations of individual heterocyclic carbenes of the іmіdazole and triazole series, as well as their fused analogues. The catalytic activities of these carbenes, and their derivatives, in some typical organic reactions (transesterifіcation and benzoin condensation) will be discussed, as will carbene complexes of transition metals (haloarene hydrodehalogenation and reduction of multiple bonds).…”

Highly efficient carbene and polycarbene catalysis of the transesterification reaction

“…[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] The known carbenes 1a, 2d, 3c, 7 were used in an isolated state. Methods for their preparation are also given in the SI.…”